Philippine police have shot dead a suspect in the 2009 killings of 57 people, the country's worst political massacre, a senior officer said on Saturday.

Maguid Amil resisted arrest after being approached by policemen in the strife-torn southern province of Maguindanao on Friday,
hurling a grenade and firing a pistol at them, senior superintendent Melencio Mina said.

The grenade did not explode but officers fired back, killing Amil, he added.

Elsewhere in Maguindanao on Friday, another suspect, Nasser Guia, was arrested by police acting on a tip-off, Mina said.

Both Guia and Amil were allegedly part of the private army of the powerful Ampatuan clan who stopped a rival political group's convoy and herded dozens to an isolated hillside where they were murdered.

"Both Amil and Guia were among those who flagged down the convoy in November, 2009 and were among those who herded the victims to... the massacre site," said Mina, quoting eyewitnesses.

The killing of members of the rival clan, lawyers, journalists and even by-standers, was believed intended to prevent a rival candidate from running against an Ampatuan in elections in May 2010.

The massacre shocked the nation, forcing then president Gloria Arroyo to crack down on her former allies, the Ampatuans. Seventy-five suspects, including key clan members are now on trial for the mass killing.

However more than three years later, dozens of other suspects remain at large, raising fears they will intimidate witnesses while maintaining the Ampatuans' influence in Maguindanao.

The trial is seen as a test of whether the Philippines can abolish the "culture of impunity" surrounding powerful figures who feel they can commit crimes without fear of punishment.

Government lawyers and human rights advocates warn that the trial could take years due to delaying tactics by some of the wealthy defendants and the overburdened legal system.